Boston (AP)  -- Sen. John Kerry on Friday debated his first Democratic primary challenger since being elected 24 years ago and again found himself defending his vote to support the war in Iraq.

The topic came up quickly in the debate at the WBZ-TV studios with Gloucester attorney Ed O'Reilly, who says his run was inspired by outrage over Kerry's 2002 vote to authorize President Bush to take military action in Iraq.

WBZ's Gregg Jensen has more.

O'Reilly accused Kerry of being ill-prepared for the vote because he didn't read a national intelligence estimate about Iraq. He also suggested Kerry's vote was a political calculation to keep his presidential prospects viable, and added that the senator's current anti-war position is also fueled by politics.

"I suggest to you ... that John Kerry changed to become an anti-war vote when the political climate changed in this country," O'Reilly said.

Kerry countered that he prepared extensively for the vote, meeting with CIA teams, top intelligence officials at the Pentagon, the National Security Council and former Secretary of State Colin Powell, among others.

"I did something better than read (the estimate)," Kerry said. "It really shows Ed's lack of understanding of what due diligence is in Washington."

Kerry said his mistake was believing the Bush administration would responsibly handle the Congressional authorization of force.

The 2004 Democratic nominee for president also denied any political motives for his decision.

"With respect to that kind of a vote ... you just don't do that based on any kind political consideration," he said. "I voted based on the security interests of our country."

The half-hour debate was moderated by WBZ-TV political analyst Jon Keller and was to air Sunday morning. Reporters were allowed to watch the debate on closed-circuit television.

It's the only debate scheduled between Kerry and O'Reilly before the Sept. 16 primary, though O'Reilly has pushed for more.

O'Reilly took his one opportunity to go on the offensive, questioning Kerry's influence and willingness to help other Democrats. He belittled Kerry's chairmanship of the Senate Committee on Small Business and Entrepreneurship, which he said was rated the second-least influential Senate committee.

O'Reilly asked whether Kerry would serve his entire six-year term. Kerry has been mentioned as a possible Secretary of State in an Obama administration.

He also hit Kerry for keeping $15 million in his campaign account after losing the 2004 presidential election, instead of distributing it to other Democrats to help them win re-election.

"Proportionally speaking ... he is the least generous of all the senators in giving around the country, when you look how much he has, in relation to how much he's given to other senators," he said.

Kerry said that charge was "plain, flat, untrue," citing $14 million he's recently raised for other Democrats, and $30 million he gave to the Democratic National Committee.